Increasingly, resources such as heavy crude oils, bitumen, tar sands, shale oils, and hydrocarbons derived from liquefying coal are being utilized as hydrocarbon sources due to decreasing availability of easily accessed light sweet crude oils. These resources are disadvantaged relative to light sweet crude oils, containing significant amounts of heavy hydrocarbon fractions such as residue and asphaltenes, and often containing significant amounts of sulfur, nitrogen, metals, and/or naphthenic acids. The disadvantaged crudes typically require a considerable amount of upgrading, for example by cracking and by hydrotreating, in order to obtain more valuable hydrocarbon products. Upgrading by cracking, either thermal cracking, hydrocracking and/or catalytic cracking, is also effective to partially convert heavy hydrocarbon fractions such as atmospheric or vacuum residues derived from refining a crude oil or hydrocarbons derived from liquefying coal into lighter, more valuable hydrocarbons.
Numerous processes have been developed to crack and treat disadvantaged crude oils and heavy hydrocarbon fractions to recover lighter hydrocarbons and to reduce metals, sulfur, nitrogen, and acidity of the hydrocarbon-containing material. For example, a hydrocarbon-containing feedstock may be cracked and hydrotreated by passing the hydrocarbon-containing feedstock over a catalyst located in a fixed bed reactor in the presence of hydrogen at a temperature effective to crack heavy hydrocarbons in the feedstock and/or to reduce the sulfur content, nitrogen content, metals content, and/or the acidity of the feedstock. Another method to crack and/or hydrotreat a hydrocarbon-containing feedstock is to disperse a catalyst in the feedstock and pass the feedstock and catalyst together with hydrogen through a slurry-bed, or fluid-bed, reactor operated at a temperature effective to crack heavy hydrocarbons in the feedstock and/or to reduce the sulfur content, nitrogen content, metals content, and/or the acidity of the feedstock. Examples of such slurry-bed or fluid-bed reactors include ebullating-bed reactors, plug-flow reactors, and bubble-column reactors.
Conventional hydrocracking processes typically use a catalyst having acidic properties and substantial hydrogen flow rates to crack and hydrogenate crude oils or heavy hydrocarbon fractions. Typical conventional hydrocracking processes produce a hydrocarbon product having relatively few mono- and di-aromatic hydrocarbons since these hydrocarbons are typically hydrogenated to naphthenic compounds which may also undergo ring opening reactions to produce alkanes. Typical conventional hydrocracking processes also produce relatively large quantities of polyaromatic hydrocarbons by annealation.
Formation of a hydrocracked composition containing a relatively large amount of mono-aromatic and di-aromatic hydrocarbon compounds relative to polyaromatic compounds containing three or more aromatic rings is desirable, particularly in the kerosene fraction of the hydrocracked composition, since the mono- and di-aromatic hydrocarbon compounds provide high energy density and reduce the smoke point of the composition when used as fuel relative to polyaromatic compounds. Furthermore, a hydrocracked composition containing a relatively large amount of mono- and di-aromatic hydrocarbon compounds relative to polyaromatic compounds is desirable since a relatively large proportion of mono- and di-aromatics to polyaromatics in a hydrocarbon composition provides a freeze point that is lower than a composition containing more polyaromatic hydrocarbons, and a low freeze point is desirable for fuels that may be made from the composition.
Cracking heavy hydrocarbons involves breaking bonds of the hydrocarbons, particularly carbon-carbon bonds, thereby forming two hydrocarbon radicals for each carbon-carbon bond that is cracked in a hydrocarbon molecule. Numerous reaction paths are available to the cracked hydrocarbon radicals, the most important being: 1) reaction with a hydrogen donor to form a stable hydrocarbon molecule that is smaller in terms of molecular weight than the original hydrocarbon from which it was derived; and 2) reaction with another hydrocarbon or another hydrocarbon radical to form a hydrocarbon molecule larger in terms of molecular weight than the cracked hydrocarbon radical—a process called annealation. The first reaction is desired, it produces hydrocarbons of lower molecular weight than the heavy hydrocarbons contained in the feedstock—and preferably produces naphtha, distillate, or distillable gas oil hydrocarbons. The second reaction is undesired and leads to the production of high molecular weight hydrocarbon compounds—often polyaromatic compounds—as the reactive hydrocarbon radical combines with another hydrocarbon or hydrocarbon radical. Furthermore, the second reaction is autocatalytic since the growing high molecular weight compounds produced by annealation are reactive with more than one cracked hydrocarbon radical. Hydrocarbon-containing feedstocks having a relatively high concentration of heavy hydrocarbon molecules therein are particularly susceptible to producing a highly polyaromatic product upon hydrocracking due to the presence of a large quantity of high molecular weight polyaromatic hydrocarbons in the feedstock with which cracked hydrocarbon radicals may combine to form larger polyaromatic hydrocarbons.
Improved hydrocarbon compositions containing a high ratio of mono-aromatic and di-aromatic hydrocarbon compounds to polyaromatic hydrocarbon compounds that may be derived from cracking heavy hydrocarbon-containing feedstocks are desirable.